A semiconductor element, in which transistor devices and diode devices such as an RC-IGBT, a MOS transistor, and a diode with a MOS gate are formed on the same semiconductor substrate, and conduction electrodes (a collector and an emitter, or a drain and a source) of the transistor devices and conduction electrodes (a cathode and an anode) of the diode devices are common electrodes, is disclosed (refer to Non Patent Literature 1). When such a semiconductor element is used as a switching device in a power conversion device such as an inverter or a converter, the semiconductor element requires a reduced switching loss and/or a reduced conduction loss.
The power conversion device has a half-bridge circuit as a basic configuration, and performs AC to DC voltage conversion, DC to AC voltage conversion, or boosts or bucks an input voltage by turning on and off a lower-arm semiconductor element and an upper-arm semiconductor element complementary to each other. In the half-bridge circuit, a dead time is set to simultaneously turn off the upper semiconductor element and the lower semiconductor element so as to prevent the occurrence of a short circuit in a power supply (arm short circuit).
A load current flows back to a diode device of one of the semiconductor elements during the dead time. When the other semiconductor element turns on after the dead time ends, the flow of a load current is switched to the other semiconductor element from the diode device. At this time, a reverse recovery current flows due to the discharging of carriers accumulated in the diode device. The reverse recovery current is a cause of increasing a switching loss and creating noise.
In contrast, Non Patent Literature 1 discloses a method by which a positive gate drive voltage is applied to the one semiconductor element slightly before the other semiconductor element turns on. According to this method, the electron current of the semiconductor element increases, a hole current decreases, the injection of holes is restricted, and thus, it is possible to reduce the reverse recovery current.
In contrast, the semiconductor element has a characteristic in which when the gate drive voltage is applied to the semiconductor element in a state where the current flows through the diode device, a channel is formed, and the injection of holes is restricted such that a conduction loss increases. In contrast, there is proposed drive control by which it is determined whether or not a current flows through the diode device, the gate drive voltage is shut off when the current flows through the diode device, and the gate drive voltage is applied when the current does not flow through the diode device.
The method disclosed in Non Patent Literature 1, by which the injection of carriers is restricted by temporarily applying the gate drive voltage (gate drive pulse) to the semiconductor element, is effective in reducing the reverse recovery current. Since it is necessary to apply the gate drive pulse during a transient period for which the flow of a current is switched between the two semiconductor elements of the half-bridge circuit, when the application of the gate drive pulse is slightly delayed, an arm short circuit occurs. In contrast, when the gate drive pulse is applied earlier, the amount of holes to be re-injected after the application of the gate drive pulse ends increases, and the effect of reducing the reverse recovery current decreases. In Non Patent Literature 1, the application timing of the gate drive pulse or a pulse width is not specifically described. It is necessary to establish device for applying the gate drive pulse so as to realize this method.
In contrast, the conduction loss characteristic of a semiconductor element associated with the application and shut-off of the gate drive voltage varies considerably depending on the types (an RC-IGBT, MOS transistor, and the like) of semiconductor elements. For this reason, it may not be possible to sufficiently reduce a conduction loss according to determination criteria in the related art to which it is determined whether or not a current flows through the semiconductor element in a forward direction of a diode device.